Microwave ovens use microwave energy to heat or cook food in a fraction of the time needed to cook with conventional ovens. Unlike conventional ovens, a microwave oven heats food without applying the external heat i.e. any flame or direct heat sources. The microwaves readily pass through many materials, such as glass, most plastics, paper and china clay, with little or no effect. Generally, these materials are used for making utensils which can be used for cooking in a microwave oven.
In microwave cooking, the metal utensils do not allow penetration of microwaves to generate the heat energy required for cooking the food, and thereby cooking efficiency is greatly reduced. Further there is also possibilities of damage for the microwave oven due to the arc formation between the metallic utensils and the door of the oven and may also results in the discoloring of cooking food which leads to a hygienic problem.
Nowadays the use of number of microwave oven in public has grown at very high rate. There has been an increase in demand for specially designed and configured cooking vessels for use in microwave ovens to provide easy and convenient way of preparing foods in microwave ovens. Many attempts have been made in the manufacture of various types of cooking vessels, casserole dishes, various types of bowls, pressure cookers for microwave and the like, to be compatible with use in microwave ovens.
U.S. Pat. No. 4,796,776 discloses a pressure cooker for use in microwave ovens includes a microwave permeable bowl-shaped container, a microwave permeable dome cover, and a steel locking ring permanently attached to the cover for maintaining the container and cover in fluid-tight seal during use. A safety valve, formed of flexible tubing, encounters a projection of the container and is deformed into a closed configuration responsive to movement of the cover, relative to the container, toward a locking position. A pressure regulator valve, consisting of a metal core surrounded by an elastically deformable casing, which is positioned over a pressure control opening in the cover to maintain a selected operating pressure level in the pressure cooker.
U.S. Pat. No. 4,853,509 discloses a rice cooker for preparation of shaped foods of rice in a microwave range is made of microwave permeable synthetic resin such as polypropylene and internally provided with mutually separate confined spaces of a fixed capacity and adapted for accommodating rice with water. As the rice cooker containing uncooked rice with water is heated in the microwave range, compression acts on the boiled rice due to pressure contact of the boiled rice swelled and water with walls defining the spaces and the boiled water is automatically shaped in the spaces. Absence of direct contact of the rice with user's hand during the process enables preparation of shaped foods of rice such as rice balls without any keen pain and blemish on the hands.
U.S. Pat. No. 5,229,563 discloses a pressure cooker and a process for making the same, wherein the pressure cooker includes a pressure container for accommodating food to be cooked, a cover for covering the container in an air-tight manner, a pressure control device for adjusting the internal pressure in the container. The container or the cover or both are partly or wholly made of a composite resin consisting essentially of polyphenylene sulfide resin and 30 wt. % to 50 wt. % glass fiber.
U.S. Pat. No. 5,310,981 discloses a microwave cooker capable of cooking an assortment of foods by a variety of methods while maintaining constant pressure. This pressure cooker boils and steams foods continuously without pressure build up and without the need to shut off the microwave oven. Food spattering and spillage are prevented by a unique internal baffle rib design and a mechanism that releaseably locks the top cover. The top cover and inner cover of the pressure cooker have the same outer diameter and each have a plurality of vent holes or apertures. Both the top cover and inner cover are positioned within container and each have a skirted edge. The skirted edges form an enclosure within the container member, specifically designed to obstruct and contain escaping food particles.
The existing microwave pressure cookers have metal parts for restraining structural changes. Moreover they have metal based deadweight pressure control systems. It is advisable to restrict the use of metallic materials inside microwave ovens because metals reflect microwave and modify the pattern of microwave distribution within the microwave chamber.
In addition, any sharp projections in the metal parts arising out of normal handling during the lifetime of the cooker can cause sparking in the chamber.
Further, the mechanical attachment between the metal parts and the polymeric parts is not perfect. This can lead to loosening of the said parts during the repeated thermal cycling which is common in normal use.
Existing microwave pressure cookers have various metallic parts like the use of metal based dead weight pressure regulators assembly, metal locking ring, metal latching members etc. Further they also permit emission of foamy liquid into the microwave chamber when froth generating foods are cooked.
In most of the prior art, the pressure cookers for microwave oven which either partially or wholly describes the apparatus of cooking system comprising container and cover that have been constructed using microwave compatible material. Further many attempts have been made in prior art to achieve a microwave compatible pressure cooking system, but still suffers from drawbacks of using some metallic parts like metal springs, metallic ring, metallic core for dead weight pressure regulating system etc., which are more dangerous and have disadvantages.